Toughened cyanoacrylate adhesives containing alkene-acrylate copolymers and method for production

ABSTRACT

A toughened cyanoacrylate adhesive composition including a cyanoacrylate monomer and an elastomeric copolymer soluble in the monomer, the copolymer being the reaction product of an olefin C 2-20  and a (meth)acrylate ester. The invention further discloses novel elastomeric copolymers which do not contain additives or functional groups which can interfere with the cure rate or stability of the adhesive composition in which they are included. Benefits of the copolymers include improved toughness of the cured adhesive composition as measured by Dynamic Mechanical Analysis.

FIELD OF THE INVENTION

The present invention is directed generally to toughening copolymeradditives such as alkene-(meth)acrylate ester copolymer additives whichwhen incorporated into adhesive compositions, such as cyanoacrylateadhesive compositions, increase the toughness of the adhesive upon cure.

BRIEF DESCRIPTION OF RELATED TECHNOLOGY

Cyanoacrylates are highly reactive monomers that undergo rapid anionicpolymerization reactions initiated by minute amounts of basic ornucleophillic species. It is generally known that small amounts ofacidic or electrophilic species will retard or inhibit this reaction. Asa consequence of their extreme anionic reactivity, commercialformulations of cyanoacrylate monomers usually contain small amounts ofacidic stabilizers that are intended to be sufficient to ensure areasonable shelf-life for the product, but not so excessive as to renderthe product inactive when it is applied to the surface of a substrate.If too little stabilizer is added, the product will be prone topremature polymerization and if too much is added it will be less activeand function less effectively as an adhesive. The commercially availableethylene/methyl acrylate toughening additives, are also problematic inthis respect because they often contain small amounts of materials, e.g.acids, that result in the over- or under-stabilization of the totaladhesive composition.

While cyanoacrylate adhesives are useful for many applications, theyinherently lack sufficient toughness and are often too brittle forcertain applications. Attempts have been made to eliminate post-cureembrittlement, through the addition of various types of additives, andparticularly toughening additives, which generally have elastomericproperties. For example, copolymers formed from the copolymerization ofacrylate esters with olefins have been added as modifiers tocyanoacrylate adhesive compositions to impart toughening properties andlower brittleness of the cured product. In particular, ethylene-methylacrylate copolymers are sold commercially by DuPont under the trademarkVamac and have been used as toughening additives for cyanoacrylateadhesives.

Loctite Corporation's U.S. Pat. No. 4,440,910 discloses a cyanoacrylateadhesive composition which contains a monomeric ester of 2-cyanoacrylicacid and about 0.5% to about 20% by weight of an elastomeric polymerselected from the group consisting of elastomeric copolymers of a loweralkene monomer and (i) acrylic acid esters, (ii) methacrylic acid estersor (iii) vinyl acetate. Acrylic rubbers disclosed in this patent includeethylene-methyl acrylates under the trade name Vamac N-123 and VamacB-124. These rubbers, as well as other Vamac products, for example,Vamac-G and Vamac-D, either contain free carboxylic acid functionalitiesand/or impurities which cause a slowing of the cure rate ofcyanoacrylate adhesive compositions or a decrease in shelf-life whenincorporated therein.

It is generally accepted in the reactive adhesive art that increasedtoughness and lower brittleness is achievable if toughening additivescan be solvated by the uncured reactive monomer and subsequently undergoa phase separation from the adhesive matrix during the curing process..The ability of these additives to be solvated by the cyanoacrylatemonomer results in a demonstrable phase separation of the additiveduring polymerization of the cyanoacrylate monomer. Phase separation isgenerally accepted as a necessary condition for increased adhesivetoughness. One problem with some commercially available ethylene-methylacrylate copolymers is that they are only partially solvated bycyanoacrylate monomers. Thus, when cyanoacrylate adhesives whichincorporate these commercially available toughening additives are cured,polymerization-induced phase separation, and hence toughening, is notoptimized.

Compatibility of the toughening additive with the cyanoacrylate monomeris an important feature. For example, as noted above the tougheningagent itself should not be so acidic that it significantly slows downthe cure rate, and not so basic or nucleophilic that it curesprematurely. Known commercially available ethylene-acrylate copolymersare also problematic in this respect because they often contain traceamounts of carboxylic functional groups, which are known to cause aslowing of the cure rate of the adhesive In addition, thesefunctionalities contribute to reduced activity upon storage.

Commercially available toughening copolymers used in cyanoacrylateadhesive formulations are generally prepared from olefin monomers havingthree carbons or less. In general, for formation of the copolymers,olefins with greater than three carbons are difficult to polymerize withalkyl acrylate esters by free radical polymerization due to their lowreactivity toward free radicals and their increased tendency to undergoa chain transfer rather than a propagation reaction.

U.S. Pat. No. 3,183,217 discloses a process for copolymerizing an alkenewith a (meth)acrylic acid ester. The '217 patent discloses higheralkenes such as 1-hexene, as well as lower alkenes, as being usefulmaterials for copolymerization with (meth)acrylic acid esters. Thispatent discloses admixing the alkenes with (meth)acrylic acid esterswith equimolar amounts of a Lewis acid per mole of the polar vinylmonomer, i.e., (meth)acrylic acid ester, and copolymerizing theresulting admixture in the presence of a free radical initiator, underanhydrous conditions and at a temperature of about −78° to about 175° C.Using this method and the ratio of reactants disclosed therein, however,limits the amount of olefin which can be incorporated into the finalcopolymer product.

It is generally desired to have higher levels of olefin relative to(meth)acrylic acid ester incorporated into the copolymer product inorder to avoid an unwanted plasticization of the cured adhesive. If theolefin incorporated into the copolymer product is too low, aplasticization rather than toughening of the cured adhesive occurs,which would be manifested by an incomplete or absence of phaseseparation of the copolymer from the adhesive matrix during cure.Moreover, the optimal amount of olefin for incorporation into thecopolymer for adhesive toughening may change depending on thecyanoacrylate monomer used in the adhesive composition.

It would therefore be desirable to have a means of varying the olefincontent in the olefin/(meth)acrylate ester copolymers in accordance withthe cyanoacrylate monomer chosen in order to control the balance betweentoughening and plasticization. Thus, there is a need for a process ofpolymerization of an alkene (olefin) with a (meth)acrylate ester whereinthe reaction conditions can be varied in order to increase the olefincontent and vary the molecular weight of the resulting copolymer toachieve adhesive toughening.

It would also be beneficial to achieve toughening without the reactivityand stabilization difficulties associated with commercial tougheningcopolymer additives. Therefore, there is a need for curable adhesivescontaining toughening copolymer additives with improved solubility incyanoacrylate monomers, as well as copolymers which do not contain traceamounts of interfering functional groups, additives, or stabilizers thatcan shorten the shelf life of the adhesive monomer or reduce itsactivity.

It would therefore be desirable to provide toughening copolymers whichhave been synthesized de novo from olefin and (meth)acrylate estermonomers by a method wherein the resulting copolymer products are freeof interfering functional groups, additives or stabilizers. Moreover, itwould be desirable to provide a method for modifying existing commercialolefin (meth)acrylate and olefin/alkenoic acid copolymers in order toeliminate acidic functional groups which are known to affect the curerate of cyanoacrylate monomers, and by so doing make them suitable foruse as toughening additives of cyanoacrylate adhesives.

SUMMARY OF THE INVENTION

The present invention provides new copolymer toughening additives whichare substantially free of acidic functionality, and acidic or basicimpurities and which are the reaction of an olefin monomer containingbetween 2 to 20 carbon atoms (olefin C₂₋₂₀) and a (meth)acrylate ester.These copolymer toughening additives are made by new processes. Oneprocess includes copolymerizing a (meth)acrylic ester with an olefinC₂₋₂₀ by (i) admixing a (meth)acrylic ester, a greater than equimolaramount of a Lewis acid per mole of the (meth)acrylic ester, a freeradical initiator and an olefin C₂₋₂₀; (ii) and heating the resultantadmixture at a temperature from about 60 to about 80° C. for a timesufficient to permit copolymerization of the ester with the olefin. Itmay, under certain circumstances, be desirable to use a solvent tofacilitate this reaction. Zinc chloride is an example of a desirableLewis acid useful in the present invention. The molar ratio of olefin to(meth)acrylic ester is desirably from about 0.1 to about 10.

An additional process for preparation of copolymer toughening additivesincludes esterification of existing olefin/(meth)acrylate orolefin/alkenoic acid copolymers containing acidic functional groups toallow conversion of the undesirable, interfering acidic functionalitiesinto esters, thus making the copolymers more suitable for use asadditives for cyanoacrylate adhesives. The esterification processincludes reacting the copolymer with an alcohol in the presence of acatalytically effective amount of an acid catalyst, at a temperature ofabout 50 to about 180° C. and in the presence of a solvent which iscapable of dissolving the copolymer and which is generally suitable foresterifications. The reaction is allowed to proceed for a timesufficient to allow conversion of the interfering carboxylic acidfunctionalities into esters. It is usually desirable to remove the acidcatalyst after the reaction is complete by precipitating the copolymerin a solvent in which the catalyst is soluble and the copolymer isinsoluble, e.g. methanol.

The present invention also provides cyanoacrylate adhesive compositionswhich are toughened by copolymers substantially free of acidicfunctional groups and acidic or basic impurities that can result in theover- or under-stabilization of the cyanoacrylate adhesive composition.

In another aspect of the invention there is provided a toughenedcyanoacrylate adhesive composition which contains at least onecyanoacrylate monomer and a copolymer toughening additive soluble in thecyanoacrylate monomer, which copolymer toughening additive issubstantially free of acidic functionalities and acidic or basicimpurities and is the reaction product of an olefin C₂₋₂₀ and(meth)acrylate ester.

In another aspect of the present invention there is provided a copolymertoughening additive which is at least partially, and desirably fully,soluble in a cyanoacrylate monomer and that, upon cure of thecyanoacrylate adhesive composition, undergoes a phase separationcharacteristic of an improvement in toughness. The present inventionfurther provides a method for preparing the copolymer tougheningadditive which allows for copolymerization of alkenes (C₂₋₂₀) with(meth)acrylate esters and provides a means to vary the degree ofincorporation of the alkene into the copolymer additive. Moreover, thepresent invention seeks to provide a method of preparing a toughenedadhesive composition that contains, as one of its components, thecopolymer toughening additives herein described.

The present invention further relates to a process for preparingcyanoacrylate adhesive compositions which when cured exhibit increasedtoughness as compared to the same or similar adhesive compositionswithout the copolymer toughener additives. This process includescombining a cyanoacrylate monomer with a toughening additive which isthe copolymer reaction product of an olefin C₂₋₂₀ and a (meth)acrylateester; subjecting the combined cyanoacrylate monomer and copolymer toconditions sufficient to allow the monomer to at least partially, anddesirably fully, solvate the copolymer. On curing, the adhesivecomposition undergoes a phase separation of the copolymer additive fromthe polycyanoacrylate matrix.

A further aspect of the present invention includes providing acomposition useful for toughening various types of adhesivecompositions, and particularly cyanoacrylate adhesive compositions,which composition includes a copolymer toughening additive which issubstantially free of acidic functionalities and acidic or basicimpurities and which is the reaction product of an olefin C₂₋₂₀ and a(meth)acrylate ester.

Finally, the present invention provides a process for sealing oradhering surfaces which includes the steps of applying the toughenedadhesive composition of the present invention to a substrate surface,placing the surface in abutting relationship with another surface andpermitting the adhesive composition to cure therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the dependence of the norbornene concentrationin the copolymer formed from norbornene and methyl acrylate on theconcentration of zinc chloride in the feed.

FIG. 2 is a dynamic mechanical analysis (DMA) trace of photocuredpoly(ethyl 2-cyanoacrylate)(PECA) without a copolymer toughening agent.

FIG. 3 is a DMA trace of photocured PECA containing 7.5% Vamac G, acommercially available toughening additive.

FIG. 4 is a DMA trace of photocured PECA containing 5%poly(hexene-co-methyl acrylate) where the mole ratio of hexene to methylacrylate is 0.27 (sample #2).

FIG. 5 is a DMA trace of photocured PECA containing 5%poly(hexene-co-methyl acrylate) where the mole ratio of hexene to methylacryl 0.21 (sample #1).

FIG. 6 is a DMA trace for UV cured CA film containing 8%poly(norbornene-co-methyl acrylate) where the molar ratio of norborneneto methyl acrylate in the copolymer is 0.37 (sample #N2).

DETAILED DESCRIPTION OF THE INVENTION

The cyanoacrylate adhesive compositions of the present invention mayemploy one or more monomeric esters represented by formulas 1, 2, and 3below:

wherein R_(a) represents a C₁₋₁₆ alkyl, alkoxyalkyl, alkylhalide,alkenyl, cyclohexyl, phenyl or furfuryl group. R_(b) may be hydrogen, aC₁₋₅ alkyl, phenyl or halogen and R_(c) may be hydrogen or methyl. TheR_(a), R_(b) and R_(c) groups can contain any linkages or substituentswhich do not adversely affect the monomer in the performance of itsintended function in the cyanoacrylate adhesive compositions. Forexample, the substituents should not adversely affect the stability orreactivity of the adhesive composition. Useful cyanoacrylate estersinclude methoxethyl cyanoacrylate and 2-chloroethyl cyanoacrylate. Inthe adhesive composition of the present invention, the esters ofcyanoacrylate can be used singly or in combination. Typically, a singleester is used having formula (1), which is selected from C₁ to about C₅alkyl, allyl and cyclohexyl esters of alpha-cyanoacrylate. The mostdesirable esters are methyl-alpha-cyanoacrylate orethyl-alpha-cyanoacrylate. Methods of preparing the monomeric esters ofalpha-cyanoacrylate are known in the art, such as those described inU.S. Pat. Nos. 2,467,927 and 3,254,111. Useful cyanoacrylate monomershaving formulas (2) or (3) include 1-cyano-1-carbethoxybutadiene-1,3,1-cyano-1-carbethoxy-3-cholorbutadiene-1,3 and1-cyano-1-carbethoxy-butene-1-yne-3. These compounds and their method ofmaking are disclosed in U.S. Pat. No. 3,316,227.

The cyanoacrylate adhesive compositions of the present invention containat least one cyanoacrylate monomeric ester and at least one copolymertoughening additive substantially soluble in the cyanoacrylate monomer.The copolymer toughening additive is desirably an elastomer materialwhich is substantially free of acidic functionalities and acidic orbasic impurities and which is the reaction product of an olefin C₂₋₂₀and a (meth)acrylate ester. Desirably, the copolymers of this inventionare completely soluble in cyanoacrylate monomer and exhibit little or nophase separation once fully dissolved in the uncured cyanoacrylatemonomer. On curing, the adhesive composition undergoes a phaseseparation of the copolymer additive from the polycyanoacrylate matrix.This is associated with an increase in toughness of the cured adhesive.

Olefins C₂₋₂₀ which are useful in making the toughening additives of thepresent invention are represented by the following monofunctional olefinstructures (4), (5), or (6):

where R₁ and R₂ may be the same or different and are independentlyselected from H, C₁-C₁₅ alkyl, cycloalkyl, alkyl ether, substituted orunsubstituted, linear or branched and R₃ may be H, C₁-C₁₀ alkyl,cycloalkyl, alkyl ether, substituted or unsubstituted, linear orbranched, X may be ═O, S or —(CH₂)_(n)—, where n may be =0-6. It isrequired that the substituents are neutral with respect to thereactivity of cyanoacrylates, i.e. non-acidic and non-basic.

The (meth)acrylate esters useful in making the toughening additives ofthe present invention are monofunctional (meth)acrylate monomersrepresented by structure (7):

where R₄ may be H or CH₃ and R₅ may be C₁-C₁₈ alkyl, cycloalkyl, alkylether, aryl, alkaryl, substituted or unsubstituted, linear or branched.It is required that the substituents do not interfere with anionicpolymerization of the cyanoacrylate monomer. Desirably, the(meth)acrylate ester used for the chain reaction which forms thecopolymer toughening additive is methyl acrylate. Other useful(meth)acrylate esters include, but are not limited to, ethyl, propyl,isopropyl, n-butyl, sec-butyl, isobutyl, t-butyl, pentyl, amyl, hexyl,4-methyl pentyl, octyl methoxy, ethyl and cyclohexyl acrylates.

In one desired embodiment, the copolymer of the present invention isobtained by a polymerization reaction of an olefin C₂₋₂₀ having at leastsix carbon atoms and a (meth)acrylate ester. In one desirableembodiment, the olefin C₂₋₂₀ may be 1-hexene or norbornene, and the(meth)acrylate ester may be methyl acrylate. For example, the reactionof norbornene and methyl acrylate, shown below, produces the copolymertoughening additive corresponding to the alternating structures shownHowever, random or block copolymers of these reactants may also beproduced.

The copolymer toughening additives of the present invention includeolefin/(meth)acrylate copolymers [poly(alkene-co-alkyl acrylate) or poly(alkene-co-alkyl methacrylate)] comprising repeat units of thesubstructure (8) shown below and one or more of the substructures (9),(10), or (11) shown below in random, alternating or block sequences. Themole ratios of (8) to (9), (10) and/or (11) may be varied from 0.1 to10.0. The molecular weights of the copolymers may be varied from about2,000 to 2,000,000 and are preferably in the range of 20,000 to 200,000.

In the above substructure examples, R₁ through R₅ are as alreadydefined. Desirably, the (meth)acrylate ester used for the condensationreaction which forms the copolymer toughening additive is methylacrylate.

The copolymer toughening additives of the present invention are at leastpartially soluble in a cyanoacrylate monomer. As already described, itis desirable to have a fully soluble rather than partially solubletoughening additive. Norbornene-methyl acrylate and hexene-methylacrylate copolymers are examples of toughening additives which arereadily soluble in monomers of ethyl 2-cyanoacrylate. It is alsoimportant that the copolymer toughening agent be insoluble in thecorresponding cured cyanoacrylate adhesive composition. Bothnorbornene-methyl acrylate and hexene-methyl acrylate undergo a phaseseparation upon polymerization (cure) of an ethyl 2-cyanoacrylatemonomer, characteristic of toughening, as determined by DMA analysiswhich provides one of the most effective means of determining the degreeof toughening potential known in the art.

The molar ratio of olefin to acrylate in the copolymers of the presentinvention can affect the characteristics of the copolymer as well as theproperties and characteristics of an adhesive in which it is used as anadditive. The present invention provides an effective means ofcontrolling the degree of incorporation of olefin in the copolymer inorder to produce a series of copolymers with different characteristicsthat can be optimized for particular applications. The present inventionemploys a method of forming copolymer toughening additives which usesthe addition of varying amounts of a Lewis acid, e.g. zinc chloride, asa means of controlling the molar ratio of alkene to acrylate in theresultant copolymer toughening additives and in promoting theincorporation of higher levels of alkene content into the resultantcopolymer toughening additives.

Olefin-acrylate copolymers of the present invention may be synthesizedfrom olefin and (meth)acrylate ester monomers by two methods which areoutlined below; a stirred reactor method and a sealed tube method. Eachof these methods was performed under anhydrous conditions and,conveniently, under ambient pressure. Moreover, each of these methodsrequires addition of a Lewis acid, e.g. ZnCl₂, and an initiator of freeradical polymerization, e.g. benzoyl peroxide. The reaction temperaturesare in the range of 60° C. for the stirred reactor method and 80° C. forthe sealed tube method, with reaction times varying from 3 to 24 hours,respectively. In general, the sealed tube method provided a higher yieldof copolymer, as compared to the stirred reactor method. It is anadvantage of these methods that no special purification of the copolymerproducts is necessary. The copolymer solution is simply washed withdistilled water to remove zinc chloride prior to drying. This is incontrast to the methods used to produce similar commercially availablecopolymers which have a comparatively high degree of impurity orundesirable moieties.

In particular, the present invention provides an improved process forcopolymerizing a (meth)acrylic ester with an olefin C₂₋₂₀. This processincludes the steps of admixing a (meth)acrylic ester, a greater thanequimolar amount of a Lewis acid per mole of the (meth)acrylic ester, afree radical initiator and an olefin C₂₋₂₀, and heating the admixture ata temperature from about 60 to about 80° C. for a time sufficient topermit copolymerization of the ester with the olefin. The mole ratio ofthe olefin to the (meth)acrylic ester in the feed is desirably fromabout 0.1:1 to about 10:1, most desirably 1:1 to about 10:1. This molarratio range has been found to give copolymer toughening additives usefulin the present invention.

One Lewis acid particularly suitable for practicing this invention isZnCl₂. Suitable olefins for purposes of this invention are thosecontaining groups promoting the release of electrons at the double bond.In one embodiment, the olefin has at least six carbon atoms. Desirableolefins to be used for copolymerization with the (meth)acrylic esterinclude hexene and norbornene. Other useful olefins include, but are notlimited to, pentene, 4-methyl pentene, octene, isooctene, 2-ethylhexene,2-methyl pentene, decene and nonene.

As previously mentioned, commercially available polymeric tougheningagents used with cyanoacrylate adhesives, including ethylene/methylacrylate copolymers sold under the trademark Vamac by DuPont, areproblematic in that they contain additives or functional groups whichcan affect the stability and fixture time of the adhesive. For example,Table 5 below shows that the effect of removing the interferingcarboxylic acid functionality in commercial Vamac G via esterificationby a process provided by the present invention is to significantlyenhance the reactivity of the cyanoacrylate adhesive and improve itsfixture time.

In particular, the invention provides a process for esterification of acommercial olefin/(meth)acrylate copolymer or olefin/alkenoic acidcopolymer, which eliminates interfering acid functionalities, thusmaking the copolymers suitable as toughening additives for cyanoacrylateadhesives. The process includes reacting the copolymer with an alcoholin the presence of a catalytically effective amount of an acid catalystat a temperature of about 50 to about 180° C. and in the presence of asolvent which is capable of dissolving the copolymer and which isgenerally suitable for esterifications for a time sufficient to convertcarboxylic acid functionalities on the commercial copolymers into esterfunctionalities. It is usually desirable to remove the acid catalystafter the reaction is complete by precipitating the copolymer in asolvent in which the catalyst is soluble and the copolymer is insoluble,e.g. methanol.

In one embodiment of the esterification process of the present inventionthe olefin/(meth)acrylate copolymer is an ethylene/methyl acrylatecopolymer, such as commercially available Vamac G, or a propylene/methylacrylate copolymer. One useful alcohol is n-butanol, although otheralcohols may be useful. However, attempts to esterify a Vamac Gcopolymer by the esterification process of the present invention wereunsuccessful when the alcohol used in the reaction was methanol. In oneembodiment, the acid catalyst is methane sulfonic acid (MSA). Otheruseful acid catalysts can include toluene sulfonic acid, sulfuric acid,phosphoric acid, as well as others generally known in the art to beuseful for esterifications, provided they can be effectively removedfrom the esterified copolymer so as not to affect the cure rate orstability of cyanoacrylate adhesive compositions into which theesterified copolymer is added as a toughener.

Suitable solvents for use in practicing the esterification process ofthe present invention are those capable of forming an azeotrope withwater which can also effectively dissolve the commercial copolymer.These include, but are not limited to, benzene, tolulene, xylene, andhigh-boiling ethers such as methyl tert butyl ether. Where tolulene isthe solvent, it is desirable that the temperature of the reaction beabout 100 to about 120° C.

The copolymers of the present invention are an improvement overcommercially available toughening agents in that they do not containinterfering additives or interfering functional groups. Instead, theyare extremely compatible with the cyanoacrylate monomer, affectingneither the storage stability of the adhesive nor the reactivity of thecyanoacrylate monomer. Stability tests performed on cyanoacrylateadhesive compositions containing the hexene/methyl acrylate copolymertoughening additives of the present invention exhibited significantimprovement over the same adhesive compositions containing certaincommercially available Vamac products. Stability tests performed oncyanoacrylate adhesive compositions containing the norbornene/methylacrylate copolymer toughening additives of the present invention alsoexhibited significant improvement over the same adhesive compositionscontaining certain commercially available Vamac products.

The copolymer toughening additive is present in amounts of about 0.5 toabout 20% by weight of the cyanoacrylate adhesive composition.Desirably, the copolymer toughening additive is present in amounts ofabout 1.5 to about 15% by weight of the cyanoacrylate adhesivecomposition. It is most desired that the copolymer be present in amountsof about 5 to about 15% by weight of the cyanoacrylate adhesivecomposition.

It is an aspect of the toughened cyanoacrylate adhesive compositions ofthe present invention that upon cure of the cyanoacrylate monomercomponent, the copolymer toughening additive undergoes apolymerization-induced phase separation. FIGS. 4-6 below show thepresence of a dispersed phase of the inventive toughening copolymerparticles in a cured cyanoacrylate adhesive composition. Thisdetermination was made by Dynamic Mechanical Analysis (DMA), generallyknown to be a good indicator of a phase separated polymer morphology,when there is a difference in glass transition temperature (T_(g))between the phases. Generally, phase-separated toughening particles havea low glass transition temperature relative to the polymericcyanoacrylate adhesive in which they are dispersed. As can be seen fromFIG. 4 of the present invention, DMA analysis indicated that thehexene-methyl acrylate copolymers made in accordance with the presentinvention (which have a hexene to methyl acrylate mole ratio of 0.27)had a T_(g) of about 9° C., as compared with 149° C. for the curedcyanoacrylate polymer. This difference in Tg is indicative of theirpotential as toughening agents for adhesives. Furthermore, when DMAanalysis was performed on a polymeric adhesive film containing theinventive norbornene-methyl acrylate copolymer toughening additives(having a norbornene to methyl acrylate mole ratio of 0.37), as shown inFIG. 6, a shoulder appeared in the scan at 96° C., which was notobserved in scans of the same polymeric adhesive without the copolymertoughening adhesive. The relative size of this shoulder compared to thealpha transition at 146° C. for the matrix of the polymeric adhesiveindicates a high degree of toughening potential for the copolymer.

The copolymer toughening additives of the present invention have as anadvantage over commercially available copolymer additives of beingsubstantially free of acidic functionalities and acidic or basicimpurities. Desirably, the inventive copolymer toughening additives arethe reaction product of an olefin C₂₋₂₀ and a (meth)acrylate ester. Inone embodiment, the olefin C₂₋₂₀ is ethylene or propylene. In anotherembodiment, the olefin has at least six carbon atoms. One desirable(meth)acrylate ester is methyl acrylate. Other (meth)acrylate esters,however, may be employed. One desirable olefin useful in the presentinvention is 1-hexene. The reaction product of 1-hexene and methylacrylate forms a particularly useful copolymer toughening agent. Wherethe olefin in the copolymer is 1-hexene and the cyanoacrylate monomer isethyl cyanoacrylate, it is desired that the mole ratio of olefin to(meth)acrylate ester is at least 0.25 or greater. It is believed that athreshold concentration of hexene in the copolymer is needed to achievethe benefits of toughening from this particular copolymer. For example,DMA analysis performed on adhesive films containing hexene-methylacrylate copolymers, as shown in FIGS. 4-5, suggested that when the moleratio of hexene to methyl acrylate was 0.21, plasticization wasprevalent, whereas when the same mole ratio was 0.27, a phase separationof the copolymer occurred, as indicated by the beta transition at 9° C.,which is associated with toughening. Thus, the relative concentrationsof the non-polar olefin and polar acrylate components of the copolymercontrol the compatibility of the copolymer in polycyanoacrylate.Copolymers having relatively high olefin contents are insoluble in thepolymerized cyanoacrylate monomer and toughen cyanoacrylate polymers,whereas copolymers having relatively high acrylate contents are solublein the polymerized cyanoacrylate monomer and plasticize materials towhich they are added. Furthermore, the threshold concentration of olefinnecessary for toughening may differ for each cyanoacrylate monomer ormonomer blend.

As described above, the reaction product of 1-hexene and methyl acrylateforms a particularly useful copolymer toughening agent. This is furtherindicated by data presented in Example 7 which show results of fracturetoughness tests performed on photocured adhesive films containinghexene-methyl acrylate copolymers. The results show a significantimprovement in fracture toughness of the formulation containing theolefin-acrylate copolymer of this invention, when compared with aformulation that does not contain any copolymer.

One desirable copolymer toughening additive is the reaction product ofnorbornene and methyl acrylate. When the molar ratio of norbornene tomethyl acrylate in the copolymer was 0.37, DMA analysis indicated a highdegree of toughening potential for the copolymer, as shown in FIG. 6,based on the presence of the shoulder at 96° C. when the cyanoacrylatemonomer was ethyl cyanoacrylate.

Another desirable copolymer toughening additive of the present inventionwhich is free of acid functionalities and acidic or basic impurities, isthe reaction product of ethylene or propylene and methyl acrylate. Asdescribed, the present invention provides a process for esterifyingcommercial olefin/(meth)acrylate copolymers, such as the ethylene/methylacrylate copolymer Vamac G, which results in the elimination of thecarboxylic acid functionalities which interfere with the cure rate ofcyanoacrylate adhesives. Thus, the present invention includesethylene/methyl acrylate and propylene/methyl acrylate tougheningcopolymers suitable for use as additives for cyanoacrylate adhesivecompositions.

While the free radical initiator used for formation of the copolymers ofthe present invention is desirably benzoyl peroxide, a diacyl peroxide,other peroxy initiators known in the art may be useful. These include,but are not limited to the following: diacyl peroxides such as dilauroylperoxide; dialkyl peroxides such as di-t-butyl peroxide and dicumylperoxide; ketone peroxides such as methylethyl ketone peroxides;peresters which readily hydrolyze, e.g., t-butyl peracetate, t-butylperbenzoate, di-t-butyldiperphthalate; and peroxycarbonates, i.e.,reaction products of isocyanates and hydroperoxides. Another usefulclass of peroxy initiators are the organic hydroperoxides such as cumenehydroperoxide, methyl ethyl ketone hydroperoxide, and t-butylhydroperoxide. Moreover, other free-radical initiators may be usefulthat do not require the presence of peroxide compounds, e.g. azonitrilesor succinic acid.

One (meth)acrylic ester useful for preparing toughening copolymers ofthe present invention is methyl acrylate, although other acrylates aspreviously described may be useful.

The inventive process for preparing a toughened adhesive compositionincludes the steps of combining the cyanoacrylate monomer with thecopolymer toughening additive, and subjecting the combined ingredientsto conditions sufficient to allow the monomer to at least partially, anddesirably fully solvate the copolymer toughening additive. In oneembodiment, the olefin C₂₋₂₀ which is copolymerized with a(meth)acrylate ester has at least six carbon atoms. In anotherembodiment, the olefin C₂₋₂₀ can have three carbons or less. Forexample, the copolymer additive can be the commercially obtainedreaction product of ethylene and methyl acrylate which has been modifiedby the esterification process provided by the present invention toremove acidic functionalities that can interfere with the reactivity ofthe cyanoacrylate monomer. It is an aspect of the foregoing process thatupon cure of the cyanoacrylate monomer the solvated copolymer undergoesa phase separation associated with a toughening of the adhesivecomposition.

It is desirable that during the step of combining the ingredients of thecyanoacrylate adhesive composition, the copolymer be present in amountsof about 5 to about 15% by weight of the cyanoacrylate adhesivecomposition. For purposes of this invention, useful particle sizes ofthe phase-separated copolymer in the cured cyanoacrylate adhesive willgenerally be in the range of about 0.2 microns to about 200 microns,desirably 2-20 microns.

In addition to the above named ingredients, the adhesive composition ofthe invention may include an inhibitor of anionic or free radicalpolymerization. The anionic inhibitor can include sulfur dioxide, sulfurtrioxide, nitric oxide, hydrogen fluoride, organic sultone inhibitors,boron trifluoride and methane sulfonic acid, which are all well known inthe art. It is an embodiment of this invention that inhibitors ofanionic polymerization be present at about 0.0001 to about 0.1% of theadhesive composition. Free radical inhibitors may include hydroquinonesor quinones which are present at about 0.0005 to about 10% of theadhesive composition.

Apart from the incorporation of initiators and polymerization inhibitorsin the compositions, it is also known and well within the contemplationof the present invention to incorporate other additives to modify thechemical and physical characteristics of the compositions. Theseadditives include viscosity modifying agents, dyes, inert fillers andplasticizers.

The invention further includes a method for sealing or adheringsurfaces. The method includes the steps of applying, to a substrate thesurface the toughened cyanoacrylate adhesive composition of the presentinvention, placing the surface in an abutting relationship with anothersubstrate surface and permitting the composition to cure. Pressure mayor may not be applied to the bond line. The toughened cyanoacrylateadhesives of the present invention generally contain an inhibitor ofanionic or free-radical polymerization. When placed on a substrate to bebonded and exposed to atmospheric and surface moisture in so doing, curegenerally occurs in a short period of time, usually in less than twominutes.

The following examples illustrate the present invention.

EXAMPLE 1 Syntheses of Olefin-Acrylate Copolymers

The olefin-acrylate copolymers of the present invention were prepared byfree-radical copolymerization of olefin and acrylate monomers or byesterification of copolymers. These copolymers were free of detectableacidic functionality or impurities. The polymerization reaction wasperformed by either of two methods, viz., in a stirred reactor or in asealed tube. The methods are described below for the synthesis ofpoly(1-hexene-co-methyl acrylate) by copolymerization of 1-hexene andmethyl acrylate and the esterification of a commercially availableethylene-methyl acrylate elastomer, Vamac G, with n-butanol. Theprocedures may be employed to prepare other copolymers by usingdifferent olefin and acrylate comonomer reactants in the polymerizationreaction or by using other olefin-acrylate elastomers and alcohols inthe esterification reaction.

Stirred Reactor Method. A 200-ml resin reaction flask was fitted with acondenser, thermocouple with temperature controller, magnetic stirrerand an oil bath. To the flask was added 39.0 g (0.465 moles) of1-hexene, 20.0 g methyl acrylate (0.233 moles) and 80 ml ethyl acetate.The mixture was stirred and 0.295 g of benzoyl peroxide (0.5% by weightof monomers) and 47.5 g (0.35 moles) of zinc chloride were added. Thelatter was added gradually; the mixture was stirred for several minutesto dissolve the zinc chloride. A gentle stream of nitrogen was blownover the mixture then it was heated to about 60-70° C. for 3 hours. Themixture was cooled, poured into a 250-ml separatory funnel, and washedtwice with about 100 ml of distilled water to remove zinc chloride. Thefunnel was allowed to stand overnight to separate the emulsion. Theorganic layer was drained and dried over anhydrous sodium sulfate thenfiltered. The solution was poured into a polypropylene beaker and placedonto a warm hot plate. When the bulk of the solvent had evaporated, thebeaker and copolymer were placed in a vacuum oven at 60° C. and 9 torrand dried over 20 hours to constant weight. A total of 12.5 g (31%yield) of poly(1-hexene-co-methyl acrylate) was recovered.

Sealed Tube Method. To a stirred solution of 14.3 g (0.105 moles) ofzinc chloride dissolved in 40 ml of ethyl acetate was added 11.6 g(0.138 moles) of 1-hexene, 6.0 g (0.07 moles) of methyl acrylate, and0.088 g of benzoyl peroxide (0.5 weight percent of comonomers). Equalamounts of the resulting mixture were added to two thick-walled 30-mlpolymerization tubes and purged with nitrogen. The tubes were sealed andheated at 80° C. for 24 hours. The resulting copolymer solution wascooled and transferred to separatory funnel. The tube was rinsed withadditional ethyl acetate (ca. 20 ml), which was also transferred to thefunnel. The solution was washed twice with 40 ml of deionized water toremove zinc chloride. The organic layer was dried over sodium sulfate,filtered, and the solvent evaporated in a vacuum oven to yield 6.08 g(52% yield) of poly(1-hexene-co-methyl acrylate).

Esterification of Ethylene-Acrylic Elastomers. Twenty grams of Vamac Gwere dissolved in 125 ml of toluene and added to a 250 mL three-necked,round bottomed flask containing a magnetic stir bar. To the mixture wasadded 0.2 g of methanesulfonic acid and 16 g of n-butanol. A condenserwith Dean-Stark trap and a thermocouple were fitted to the flask. Themixture was refluxed for 12 hours, during which time IR spectra takenperiodically to chart the disappearance of the carboxylic acid's —OHpeak. The mixture was cooled and the polymer precipitated in about 600mL of methanol. The polymer was placed in a dish and dried overnight ina vacuum oven at 60° C. About 13 g were isolated. Spectroscopic analysisshowed the compound to be a terpolymer of ethylene-methyl acrylate andN-butyl acrylate.

EXAMPLE 2 Characterization of Hexene/Methyl Acrylate Copolymers

This example was performed for the purpose of characterizing, usinganalytical techniques, the hexene/methyl acrylate copolymer tougheningadditive formed using the inventive process. The isolated copolymerswere characterized by ¹H NMR spectroscopy and size exclusionchromatography and were determined to be copolymers of the reactants, asopposed to homopolymer blends. This was confirmed further by solubilitytests and by the fact that hexene does not polymerize under thesereaction conditions. The inventive copolymer toughening additives werecompletely soluble in non-solvents of poly(1-hexene), e.g. ethyl acetateand acetone, and insoluble in solvents of poly(1-hexene), e.g. hexaneand cyclohexane. Table 1 below shows the characterization andcomposition of hexene/methyl acrylate copolymers which were synthesizedby the sealed tube method in Example 1.

TABLE 1 Characterization and Composition of Hexene/Methyl AcrylateCopolymers [ZnCl₂]/ [Hex]/ [Hex]/ Copolymer [MA] [MA] [MA] Mn sample #feed feed copolymer (PMMA) MWD CH/O 1 0 1 0.21 15,900 2.7 2.2 2 1 1 0.2717,600 2.1 2.4 3 1 2 0.46 12,700 2.3 2.9 4 1 3 0.42 12,400 1.8 2.8 5 1.52 0.50 13,600 2.1 3.0 Vamac G 60,800 4.8 4.3

The level of hexene incorporated into the copolymer was determined fromthe integral ratio of the methyl group signals of hexene and methylacrylate, δ_(0.9)/δ_(3.7), in the NMR spectrum, which provides the moleratio of the two comonomers directly, i.e. [Hex]/[MA]. Molecular weightmeasurements were determined relative to PMMA standards. The molecularweight ratio of hydrocarbon to oxygen in the copolymer (CH/O) is anarbitrary but convenient index of the degree of incorporation of olefininto the copolymer toughening additive product. The CH/O value for VamacG is approximately 4.3 and the corresponding value for poly(methylacrylate)homopolymer is 1.7. The CH/O value for poly(1-hexene) isundefined since it does not contain oxygen.

Referring now to samples 3 and 5 of Table 1, which were synthesizedusing the sealed tube method, it is clear from sample 5 that inclusionof a greater than equimolar amount of ZnCl₂ (a Lewis acid) per mole ofmethyl acrylate results in a desirably higher mole ratio of hexene tomethyl acrylate in the copolymer reaction product, as compared to sample3 where an equimolar amount of the halide per mole of methyl acrylate isused: For both samples 3 and 5, the ratio of hexene to methyl acrylatein the feed was 2.0. There is a clear correlation between the amount ofZnCl₂ added to the reaction and the amount of olefin incorporated intothe copolymer formed. In particular, the data from Table 1 shows thatolefin content in the copolymer increases with increasing concentrationsof ZnCl₂.

EXAMPLE 3 Copolymerization of Norbornene and Methyl Acrylate

In this example, norbornene is copolymerized with methyl acrylate, usingZnCl₂ as the Lewis acid to produce copolymers having random, alternatingor block structures incorporating units derived from norbornene andmethyl acrylate. For example, the scheme below shows the structure of aperfectly alternating copolymer of norbornene and methyl acrylate.

A series of poly(norbornene-co methyl acrylates) were synthesized byheating solutions of norbornene and methyl acrylate in ethyl acetate fortwo hours at 70° C., according to the stirred reactor proceduredescribed in Example 1. In all cases equimolar amounts of the twocomonomers were employed. The reaction was performed under conditions ofvarying concentrations of zinc chloride in order to produce copolymerswith different compositions. The results are presented in Table 2.

TABLE 2 Copolymerization of Norbornene and Methyl Acrylate [ZnCl2]/Yield Mn [NB]/ Sample [MA] (%) (PMMA) MWD [MA] CH/O N1 0 44 37,000 3.10.38 2.8 N2 0 42 36,000 2.8 0.37 2.8 N3 0.5 47 54,000 2.3 0.62 3.5 N41.0 37 — — 0.78 4.0 N5 2.0 34 35,000 1.7 0.85 4.2 N6 2.5 45 — — 0.95 4.5Vamac G 60,000 3.8 4.3

Referring now to Table 2, which shows the characterization andcomposition of copolymers formed from norbornene and methyl acrylateusing the stirred reactor method, it can be seen that the amount ofnorbornene incorporated into the copolymer increases linearly withincreasing concentrations of ZnCl₂ in the feed. The data presentedgraphically in FIG. 1 shows that there is a good correlation between theconcentration of ZnCl₂ in the reaction mixture and the concentration ofnorbornene incorporated into the copolymer for molar ratios of ZnCl₂ tomethyl acrylate up to 2.5. The continuous line in FIG. 1 is the dataplotted from the least squares method. When 2.5 moles of ZnCl₂ per moleof methyl acrylate is used, a copolymer (N6) containing a molar ratio ofnorbornene to methyl acrylate [NB]/[MA] of almost 1:1 is formed, asdetermined from the ¹H NMR spectrum.

Vamac G, a commercially available toughening agent, is included in Table2 for comparative purposes. Molecular weights (M_(n)) and distributions(MWD) were lower than those recorded for the commercial ethylene-methylacrylate copolymer, Vamac G. The CH/O index, which provides an index ofthe degree of incorporation of olefin into the copolymer and permitscomparisons to be made between different olefin-acrylate copolymers,indicates that sample N6 has a CH/O index which exceeds that of Vamac G,a commercially available toughening agent.

EXAMPLE 4 Dynamic Mechanical Analysis

Toughening particles dispersed in polymeric adhesives are typicallyabout 0.5 to 5 μm in diameter and are present at a volume fraction ofabout 5 to 30%. Dynamic mechanical analysis (DMA) represents one of thebest methods for detecting the presence of a dispersed phase oftoughening particles in an adhesive or polymeric matrix. In such cases,the dispersed phase is indicated by the presence of one or moresecondary transitions in the DMA plots of elastic modulus (G′), lossmodulus (G″) or damping (G″/G′, also defined as tan δ) versustemperature.

Cyanoacrylate (CA) adhesive formulations were prepared for DMA testingby dissolving the copolymers in ethyl 2-cyanoacrylate containing 25 ppmBF₃ as stabilizer. The formulae were sensitized for photocuring by theaddition of 1% Irgacure 1700 (supplied by Ciba Geigy) and 130 ppmferrocene. Films of the cured adhesive were prepared between two releasetreated overlapping glass slides. The slide assemblies were filled withthe sensitized liquid formulation by capillary action while a gap of 1mm was maintained by means of a spacer located at one end of eachassembly. The filled assemblies were exposed to ultraviolet (UV) lightfrom an Oriel Corp. Model 87331 mercury arc lamp projector (1 8J/cm²/side) to cure the adhesive. After curing, the films were removedfrom the glass plates by immersing the assemblies in warm water for afew minutes. The free films were then dried to constant weight and cutto the required dimensions. This method provided highly uniformdefect-free cyanoacrylate polymer films.

DMA were performed on a Rheometrics RDAII in torsional shear mode at afrequency of 10 rad/s (˜1.6 Hz). The analyses were conducted over thetemperature range −120 to +150° C. Cyanoacrylate adhesives containingcopolymer samples #1, #2 of Example 3 and #N2 of Example 4 were tested.For comparative purposes, films of unmodified UV cured poly(ethyl2-cyanoacrylate) (PECA) and PECA toughened with Vamac G were alsoexamined. The results are presented in FIGS. 2-6.

Referring to FIG. 2, which shows the trace for UV cured PECA withoutadded toughening agent, a single α-transition at 147° C. is observed inthe tan δ plot corresponding to the glass transition (T_(g)) for thepolymer. Above T_(g), the polymer rapidly decomposes. In contrast, theDMA traces of the cured material containing 7.5% by weight Vamac G (FIG.3), show a weak β-transition (secondary transition) at −15° C. in thetan δ plot and an associated reduction in the elastic modulus (G′). Thistransition corresponds to the Tg of the added elastomer, which hasphase-separated from the CA polymer. The main transition is stillobserved at approximately 150° C. indicating that there is littleplasticization of the CA by the added copolymer, which providesadditional evidence, that phase segregation has occurred. The film brokebefore the α-transition was completely defined. This is frequentlyobserved with CA samples since the onset of degradation occurs almostimmediately after the polymer becomes rubbery.

FIG. 4 shows the DMA traces corresponding to the PECA film containing 5%hexene/methyl acrylate copolymer sample #2 (Table 2, Example 3), whichhas a hexene/methyl acrylate mole ratio of 0.27. This trace clearlyshows a β-transition at about 9° C. corresponding to the phase separatedrubber. The corresponding modulus drop is approximately the samemagnitude as that observed with Vamac G, indicating a comparable degreeof phase separation. In addition the α-transition, at 149° C., isessentially unchanged from that of the unmodified CA polymer (FIG. 2),indicating that there is little or no plasticization of the CA polymer.Thus Vamac G and hexene/methyl acrylate copolymer #2 are similar withrespect to their insolubility in PECA and may be expected to impartsimilar degrees of toughness to the cyanoacrylate polymer.

In contrast, the CA film containing copolymer #1, having a lower moleratio of hexene/methyl acrylate ([Hex]/[MA]=0.21), does not show adistinct β-transition and the value of the α-transition is loweredcompared to that of the unmodified polymer, as shown in FIG. 5.

This suggests that there is a threshold concentration of hexene in thecopolymer (between mole ratio hexene/MA 0.21-0.27), above which phaseseparation occurs and below which plasticization is prevalent. DMA wasalso performed on a photocured ECA film containingpoly(norbornene-co-methyl acrylate), having a N/MA ratio of 0.37 (FIG.6). This film exhibits a primary transition due to the CA polymer at146° C., which is close to the value observed for unmodified PECA (FIG.2). A secondary transition appears as a shoulder at approximately 96°C., which is not observed in scans of unmodified PECA. This is likelydue to the T_(g) of the phase-separated copolymer. The relative size ofthis shoulder compared to the alpha transition indicates a high degreeof toughening potential for the copolymer, although the relatively highT_(g) may restrict its benefit to a narrow temperature range of use inCA's in the region 90-150° C. A less well-defined y-transition is alsoobserved at 17° C.

EXAMPLE 5 Stability Tests and Fixture-Time Tests of Toughened CAAdhesives.

The olefin/alkyl (meth)acrylate copolymers prepared as described inExample 1 are neutral with regard to their influence on the reactivityand stability of cyanoacrylate monomers. Compositions for stability andreactivity testing were prepared by dissolving 8% copolymer in ethyl2-cyanoacrylate (ECA) containing 25 ppm of boron trifluoride. Stabilitywas determined by the length of time that the formulated productremained in a liquid and useable state under ambient storage conditionsin a polyethylene container. Adhesives containing the inventivehexene/methyl acrylate copolymer #5 and norbornene/methyl acrylatecopolymer #N2 additives, respectively, were selected, as well as twocommercially available ethylene/methyl acrylate elastomers, Vamac G andVamac D, which were chosen for comparative purposes. The stability testresults are presented in Table 3.

TABLE 3 Stability of Toughened Cyanoacrylate Adhesives Under AmbientStorage Conditions Copolymer Stability Sample # (months) none >6Hexene/methyl acrylate #5 >6 Norbornene/methyl acrylate #N2 >6 VamacG >6 Vamac D 0

The data show that the inventive copolymers #5 and #N2 provide stableliquid adhesive compositions for periods exceeding 6 months, as does thecommercial material Vamac G. However, the related material, Vamac D,provides unstable adhesives that polymerize in a matter of a few hours,following the dissolution of the copolymer.

The reactivities of the adhesive formulations were determined bymeasuring the minimum time required to fixture a single-lap shearadhesive joint under static loading of 3 kilograms. The tests wereconducted on 1×4 inch², solvent wiped, mild-steel test specimensassembled with an overlap area of 0.5 square inches. The stress wasapplied to the adhesive for 5 seconds and the assembly was consideredfixtured if the specimens did not move relative to one another duringthat time. Three consecutive replicate measurements were made and theminimum time range recorded. The results are presented in Table 4.

TABLE 4 Fixture Times of ECA Adhesives Containing Various CopolymersCopolymer Sample Age Fixture-Time Sample # (months) (seconds) None 020-30 Hex/MA #5 0 20-30 Hex/MA #5 11 20-30 Norb/MA #N2 6 10-20 Vamac G 0 90-105 Vamac G 6 210-240

Referring to Table 4, the reactivities of adhesives containingcopolymers of the present invention were compared with the sameformulation containing Vamac G. The data show that the presence of 8%hexene/methyl acrylate has no influence on the reactivity of themonomer. The fixture time is identical to that of the unmodified monomerbefore and after aging for 11 months under ambient conditions. Withnorbornene/methyl acrylate, there is a slight activating effect, butsince this adhesive formulation has stability in excess of six months,the copolymer is acceptable as an additive for use in cyanoacrylates. Incontrast, there is a significant deactivating effect in the compositioncontaining Vamac G. The fixture-time of a freshly prepared adhesivecomposition is over three times higher than that of the unmodifiedmonomer. In addition, the reactivity is further reduced on storage, suchthat after six months the fixture time is almost an order of magnitudelonger than that of the unmodified adhesive. This material is clearlyundesirable for use as an additive of cyanoacrylate adhesives. Incontrast the copolymers of the present invention represent a markedimprovement over commercial toughening agents in terms of both fixturetime and stability.

EXAMPLE 6 Esterification of Olefin/Alkyl (Meth)Acrylate Copolymers

This example demonstrates:

(1) that the fixture time of cyanoacrylate compositions containingcommercially available copolymer toughening additives such as Vamac Gare significantly longer than those same compositions containing one ofthe inventive copolymer toughening additives of the present inventionwhich was prepared by an esterification of the Vamac G. This is believedto be due to the acidic functionality and/or impurity present in thesecommercial copolymer additives as a result of their method ofpreparation For example, elimination of the acidic functionality and/orimpurities responsible for such slow fixture times by using anesterification process of the present invention to butylate the Vamac Gcopolymer, resulted in improved fixture times (70-90 seconds) which weretwo to three times faster than an unmodified Vamac G copolymer (150-180seconds) in a test composition.

(2) a process for converting a copolymer unsuitable for use as atoughening additive in cyanoacrylate adhesives into a copolymer usefulas a toughener in cyanoacrylate adhesives by elimination of the acidfunctionalities therein which affect the reactivity of the cyanoacrylatemonomer. In particular, the process is for esterification of anolefin/(meth)acrylate copolymer or an olefin/alkenoic acid copolymercontaining carboxylic acid functionalities, which process includesreacting the copolymer with an alcohol in the presence of acatalytically effective amount of an acid catalyst, at a temperature ofabout 50 to about 180° C. and in the presence of a solvent suitable foresterification for a time sufficient to allow conversion of thecarboxylic acid functionalities into esters. The process furtherincludes substantial removal of the acid catalyst by precipitating thecopolymer in a solvent in which the catalyst is soluble, but thecopolymer is insoluble.

Certain commercially available olefin-acrylate elastomers, such as VamacG, may be used to toughen cyanoacrylate adhesives. However, theseproducts have the undesirable effect of slowing the cure speed as aconsequence of the presence of a low concentration of free carboxylicacid groups present in the copolymer. In order to overcome thislimitation, we esterified Vamac G to eliminate the free carboxylic acidfunctionality.

Vamac G was successfully esterified with excess n-butanol in thepresence of methane sulfonic acid (MSA) as catalyst to provide thecorresponding ethylene-methyl acrylate-butyl acrylate terpolymer usingthe scheme below. The product was isolated by precipitation in methanoland drying to constant weight. This has the effect of purifying thecopolymer such that the acid catalyst is substantially removed from it.

Infrared analysis of the starting Vamac G polymer shows a broadabsorption band at 3265 cm⁻¹ characteristic of a carboxylic acid group.This absorption band is absent in the spectrum of the product indicatingthat esterification is essentially complete. Acid analysis of the newmaterial was 3.8 mg KOH/g, which represents a significant reduction fromthe 21.1 value of the starting Vamac G. It confirms that theesterification is essentially complete and that the acid added tocatalyze the esterification reaction was effectively removed during theprecipitation of the copolymer. Proton NMR analysis confirmed thepresence of a partially butylated structure and showed that the degreeof butylation was in the range of 16-21% of total ester content. Thisindicates that significant transesterification has also occurred underthe above conditions where x+n=˜y/5 in this scheme. Attempts to esterifythe elastomer with excess methanol by this method were unsuccessful.

A test formulation was prepared by dissolving 8% by weight of thebutylated Vamac in ethyl 2-cyanoacrylate, which contained 50 ppmmethanesulfonic acid as stabilizer. A similar composition containing 8%unmodified Vamac G was also prepared for comparative purposes. Theadhesive fixture times were determined as described in Example 6 and theresults are presented in Table 5.

TABLE 5 Comparative Fixture Times of ECA Adhesives Containing ButylatedVamac G and Unmodified Vamac G Fixture time Toughening Agent (seconds)Butylated Vamac G 75-90 Vamac G 150-180

EXAMPLE 7 Fracture Toughness Test

This example demonstrates that the olefin-acrylate copolymers of thepresent invention are capable of toughening cyanoacrylate adhesives.

A sample of poly(1-hexene-co-methyl acrylate) containing 25 mole %hexene was prepared according to the stirred reactor method described inExample 1. Ultra-violet light (UV) curable adhesive formulations wereprepared by blending together the material components listed in Table 6.

TABLE 6 Formulations of UV-Curable Cyanoacrylate Adhesives Formula AFormula B Component weight % weight % Ethyl 2-cyanoacrylate 94.98498.984 Poly(1-hexene-co-methyl acrylate) 4.000 0 Irgacure 1700 1.0001.000 Ferrocene 0.013 0.013 Boron trifluoride 0.003 0.0003

Photocured films of each formulation were prepared according to theprocedure already described in (Example 4). The films were cut andmachined into standard sized fracture test specimens and tested forfracture toughness according to ASTM E813-89, Standard Test Method forJ_(IC), a Measure of Fracture Toughness. The results are presented inTable 7.

TABLE 7 Fracture Toughness and Failure Modes for Cyanoacrylate AdhesivesFracture Toughness Energy Release Rate Formulation lbs/in Mode ofFailure A 10.7 (±1.7) (J_(q)) Ductile B  1.5 (±0.4) (G_(q)) Brittle

Formulation A, containing a small amount of olefin-acrylate copolymer isrepresentative of the present invention, whereas formulation B isincluded for comparative purposes. It does not contain any copolymer.The results clearly show a significant improvement in fracture toughnessof formulation A compared to formulation B. The improved toughnessresulting from the presence of the copolymer is further confirmed by themode of failure. When copolymer is present, the failure occurs in aductile mode, whereas brittle failure is observed when the copolymer isomitted.

The invention being thus described, it will be evident to those skilledin the art that the same may be varied in many ways. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention and all such modifications are intended to be defined by theclaims.

What is claimed:
 1. A toughened cyanoacrylate adhesive compositionsubstantially free of acidic or basic impurities comprising: (i) atleast one cyanoacrylate monomer (ii) an elastomeric copolymer tougheningadditive substantially soluble in said cyanoacrylate monomer, saidcopolymer being the reaction product of an olefin C₂₋₂₀ and a(meth)acrylate ester and being substantially free of acidicfunctionalities and acidic or basic impurities, wherein said olefinC₂₋₂₀ has at least six carbon atoms.
 2. The composition of claim 1wherein said cyanoacrylate monomer comprises at least one of formulas(1), (2) or (3):

wherein R_(a) represents a C₁ to C₁₆ alkyl, alkoxyalkyl, alkylhalide,alkenyl, cyclohexyl, phenyl or furfuryl group; R_(b) is hydrogen, a C₁₋₅alkyl, phenyl or halogen and R_(c) is hydrogen or methyl.
 3. Thecomposition of claim 1 wherein said olefin C₂₋₂₀ comprises

wherein R₃ is H, C₁-C₁₀ alkyl, cycloalkyl, alkyl ether, substituted orunsubstituted, linear or branched; X is O, S or —(CH₂)_(n)—; providedthat R₃ does not adversely affect the reactivity of said cyanoacrylatemonomer.
 4. The composition of claim 1 wherein said (meth)acrylate esterhas the formula (7):

wherein R₄ is H or CH₃ and R₅ is C₁-C₁₈ alkyl, cycloalkyl, alkyl ether,aryl, alkaryl, substituted or unsubstituted, linear or branched, withthe proviso that substituents R₄ and R₅ do not adversely affect thereactivity of said cyanoacrylate monomer.
 5. The composition of claim 1wherein said olefin C₂₋₂₀ is 1-hexene.
 6. The composition of claim 1wherein said olefin C₂₋₂₀ is norbornene.
 7. The composition of claim 1wherein said copolymer is the reaction product of 1-hexene and methylacrylate.
 8. The composition of claim 1 wherein said copolymer is thereaction product of norbornene and methyl acrylate.
 9. The compositionof claim 1 wherein upon cure of said cyanoacrylate monomer saidcopolymer undergoes a phase separation therefrom.
 10. The composition ofclaim 1 wherein said copolymer is present in a concentration range ofabout 0.5 to about 20% by weight of the adhesive composition.
 11. Thecomposition of claim 1 wherein said copolymer is present in aconcentration range of about 1.5 to about 15% by weight of thecomposition.
 12. The composition of claim 1 wherein said copolymer ispresent in a concentration range of about 5 to about 15% by weight ofthe composition.
 13. The composition of claim 1 further comprising aninhibitor of anionic polymerization.
 14. The composition of claim 13wherein said inhibitor of anionic polymerization is selected from thegroup consisting of sulfur dioxide, sulfur trioxide, nitric oxide,hydrogen fluoride, organic sultone inhibitors, boron trifluoride,methane sulfonic acid and combinations thereof.
 15. The composition ofclaim 13 wherein said inhibitor of anionic polymerization is present atabout 0.0001 to about 0.1% by weight of said adhesive composition. 16.The composition of claim 1 further comprising an inhibitor offree-radical polymerization.
 17. The composition of claim 16 whereinsaid inhibitor of free-radical polymerization is a hydroquinone orquinone.
 18. The composition of claim 16 wherein said inhibitor offree-radical polymerization is present at about 0.0005 to about 10% byweight of said adhesive composition.
 19. A composition substantiallyfree of acidic or basic impurities useful for toughening adhesives,comprising the reaction product of an olefin C₂₋₂₀ and a (meth)acrylateester, said reaction product being substantially free of acidicfunctionalities and acidic or basic impurities, wherein said olefinC₂₋₂₀ has at least six carbon atoms.
 20. The composition of claim 19wherein the olefin C₂₋₂₀ comprises

wherein R₃ is H, C₁-C₁₀ alkyl, cycloalkyl, alkyl ether, substituted orunsubstituted, linear or branched; X is O, S or —(CH₂)_(n)—; with theproviso that R₃ does not adversely affect the reactivity of acyanoacrylate monomer.
 21. The composition of claim 19 wherein the(meth)acrylate has the formula 7:

wherein R₄ is H or CH₃ and R₅ is C₁-C₁₈ alkyl, cycloalkyl, alkyl ether,aryl, alkaryl, substituted or unsubstituted, linear or branched, withthe proviso that substituents R₄ and R₅ do not adversely affect thereactivity of a cyanoacrylate monomer.
 22. The composition of claim 19wherein said olefin C₂₋₂₀ is 1-hexene.
 23. The composition of claim 19wherein said olefin C₂₋₂₀ is norbornene.
 24. The composition of claim 19wherein said composition is the reaction product of 1-hexene and methylacrylate.
 25. The composition of claim 19 wherein said composition isthe reaction product of norbornene and methyl acrylate.
 26. Thecomposition of claim 19 wherein said composition is at least partiallysoluble in a cyanoacrylate monomer.
 27. A process for preparing acurable toughened cyanoacrylate adhesive composition, said compositioncomprising at least one cyanoacrylate monomer and a copolymer tougheningadditive substantially free of acidic functionally and acidic or basicimpurities and soluble in said cyanoacrylate monomer, said copolymertoughening additive being substantially free of acidic functionalitiesand acidic or basic impurities and being the reaction product of a(meth)acrylate ester and an olefin C₂₋₂₀, said process comprising thesteps of: (a) combining said cyanoacrylate monomer with said copolymertoughening additive; and (b) subjecting said combined ingredients toconditions sufficient to allow said cyanoacrylate monomer tosubstantially solvate said copolymer.
 28. A process for copolymerizing a(meth)acrylic ester with an olefin C₂₋₂₀, wherein said olefin C₂₋₂₀ hasat least six carbon atoms the copolymerized product being substantiallyfree of acidic functionality and acidic or basic impurities, saidprocess comprising the steps of: (a) admixing a (meth)acrylic ester, agreater than equimolar amount of a Lewis acid per mole of said(meth)acrylic ester, a free radical initiator and an olefin C₂₋₂₀wherein said olefin C₂₋₂₀ has at least six carbon atoms, and (b) heatingat a temperature from about 60 to about 80° C. for a time sufficient topermit copolymerization of said ester with said olefin, the mole ratioof said olefin to said (meth)acrylic ester being from about 0.1 to about10.
 29. A process for esterifying an olefin/(meth)acrylate copolymer orolefin/alkenoic acid copolymer, the esterified product beingsubstantially free of acidic functionality and acidic or basicimpurities, said process comprising the steps of: (a) reacting saidolefin/(meth)acrylate copolymer or said olefin/alkenoic acid copolymerwith an alcohol in the presence of a catalytically effective amount ofan acid catalyst at a temperature of about 50 to about 180° C. and inthe presence of a solvent which is capable of dissolving said copolymerand which is suitable for esterification processes for a time sufficientto convert carboxylic acid functionalities on said copolymer into esterfunctionalities; and (b) removing said catalyst from said copolymerfollowing esterification.
 30. The process of claim 29 further includingthe step of removing any acidic or basic impurities.
 31. A process forsealing or adhering surfaces which comprises: (a) applying to at leastone of said surfaces the adhesive composition of claim 1; and (b)placing said surfaces in an abutting relationship until said compositionhas cured.